I'm using Teamviewer to help my family members with their computer problems. Sometimes they use the mouse and keyboard when I'm in the middle of something important. I don't want to tell them not to do it, instead I want to block their keyboard and mouse input when I'm connected to their machine.

I couldn't find an option to do this in Teamviewer preferences. Does anybody know how to do it?

You do this from the remote computer after you have connected. When using Windows on the remote computer, Click on the "Actions" tab (Gear) at the top of the screen and select "Disable Remote Input". When remoting in from an Ipad, Click on the "Actions" (wrench) tab at the bottom of the screen and select "Disable Remote Input". I have not used Teamviewer on a Mac or IPhone so I am not sure of the exact steps. But it still should be under the "Actions" Menu.